Contemporary hard disk drives include an actuator assembly pivoting through an actuator pivot to position one or more read-write heads, embedded in sliders, each over a rotating disk surface. The data stored on the rotating disk surface is typically arranged in concentric tracks. To access the data of a track, a servo controller first positions the read-write head by electrically stimulating the voice coil motor, which couples through the voice coil and an actuator arm to move a head gimbal assembly in positioning the slider close to the track. Once the read-write head is close to the track, the servo controller typically enters an operational mode known herein as track following. It is during track following mode that the read-write head is used to access data stored in the track.
Micro-actuators provide a second actuation stage for positioning the read-write head during track following mode. They often use an electrostatic effect and/or a piezoelectric effect to rapidly make fine position changes. They have doubled the bandwidth of servo controllers and are believed essential for high capacity hard disk drives from hereon.
Using micro-actuator requires an accurate stroke sensitivity estimate. The stroke sensitivity is the displacement of the read-write head in the lateral plane for a given electrical stimulus. There are several difficulties associated with achieving this. The stroke sensitivity often needs to be measured on an individual basis, inside the assembled hard disk drive, during access operations. The stroke sensitivity measurements may need to be repeated as the hard disk drive ages and may differ for each of the micro-actuators and their coupled read-write heads.
There is also a question as to whether and how much a specific micro-actuator is aiding the track following process. One useful estimate of its contribution would be an effective estimate of its operational bandwidth, over which there is close to flat frequency response.
Finally, there is the need to calibrate each specific micro-actuator as to the details of its dynamics, including mode peaks, possibly related to air flow turbulence or other sources of mechanical vibration affecting the micro-actuator.